Gearing Lingo

So you tossed a set of 4.10s in your rear axle 'cause you want to go fast...but do you really have any idea what you're doing? According to our reader survey, plenty of you are willing to admit that you really don't.

That's why this story will cover gearing basics for first-timers as well as share some of the physics of gearing that you hard-core types may not have considered. We'll also draw on some of our own experiences to help you decide how to gear your car for the street or strip. It may help if you read the "Gearing Lingo" in the right column first.

What Is a Gear Ratio?

When you hear people refer to numbers like 3.08, 3.73, or 4.10, they're talking about the ratio of the ring-and-pinion gears in the rear axle—hence, the numbers are more accurately 3.08:1, 3.73:1, or 4.10:1. The ratio is the number of teeth on the driven gear (ring) divided by the number of teeth on the drive gear (pinion). So, if the ring gear has 37 teeth and the pinion has 9 teeth, the ratio is 4.11:1. That also means that for every one turn of the ring gear, the pinion will turn 4.11 times.

What Do Gears Do?

In addition to changing the direction of power flow by 90 degrees (from the driveshaft to the axles), the purpose of the rearend gears is to multiply the torque delivered by the engine and transmission. Gears can be thought of as complex levers. In other words, they provide a mechanical advantage that multiplies work—in this case, torque—to help the engine's power move the vehicle. Lower gears are like a longer lever: They provide more mechanical advantage. Higher gears are like a shorter lever: They provide less mechanical advantage. It's similar to when you use a long breaker bar instead of a short ratchet handle to remove tight lug nuts. Just like a long bar puts more torque on a lug nut, lower axle gears provide more torque to the wheels.

It's very easy to calculate the torque multiplication provided by your axle gears—just multiply by the gear ratio. For example, let's assume that the engine and transmission are delivering 100 lb-ft of torque to the pinion gear. If the gear ratio of the ring-and-pinion is 4.10:1, then the output torque is 410 lb-ft (100x4.10). Similarly, if the gear ratio is 3.08:1, then the output torque will be 308 lb-ft. It's easy to see that the lower 4.10:1 gears put more power to the ground than the higher 3.08:1 gears. Keep in mind that the engine's power has not changed but that the available torque to the tires has.

Axle Gears vs. Engine RPM

Considering that lower gears provide greater torque multiplication, it would seem that they are always the best choice for performance use. However, lower gears require more input speed (engine rpm) to produce the same output speed (tire rpm). Higher gears multiply torque less, but they require less input speed to deliver the same output speed; that's why axle ratios also determine engine cruise rpm.

Again, think of a long breaker bar versus a short ratchet. With your hand at the far end of a breaker bar (which is a longer lever, like lower gears) the job is much easier, but to turn a lug nut one complete revolution requires your hand to travel a much greater distance than it would with a smaller wrench (shorter lever, like higher gears) which has a smaller turning circle. Similarly, lower axle gears (longer lever) require the engine to move a greater distance (turn more times) per tire revolution than higher gears. To look at it a different way, if the trans is in a gear with a 1:1 ratio (like Fourth gear on most four-speeds) and the rear gears are 3.08s, then the engine must turn 3.08 times for every one rotation of the tires. Lower 4.10:1 gears would make the engine turn 4.10 times for each turn of the tire, so lower gears cause higher engine rpm at any road speed.

Axle Gears vs.Vehicle Performance

If you continue to visualize the example of removing lug nuts with the long or short wrench, it will help you understand how to gear your car for quarter-mile acceleration. Anything you can do to reduce the resistance of the lug nut turning will allow you to either use a shorter wrench or less effort; similarly, the lighter the car, the less the effort required to move it, the higher the gears you can get away with. Also, your musclebound buddy can probably loosen a lug nut with a short wrench, whereas a weakling requires a longer wrench; it's the same on a car—the more input power (engine torque), the higher the gear you can use.

In the real world, typical street machines with aspirations for good dragstrip performance generally run quickest with 4.10:1 gears. Lower gears are required if the car is very heavy, or if the engine makes its power at the upper end of the rpm scale. Sometimes higher gears are used if the engine has gobs of low-end torque and doesn't like to spin at high rpm—455 Buicks, Olds, and Pontiacs are perfect examples of cars that can run quick with 3.50:1 or 3.73:1 gears. Also, nitrous allows the use of higher gears, not only because it increases torque dramatically, but because it causes engine rpm to increase at a rapid rate. Add nitrous and you may find yourself shifting earlier, and if you're in top gear at half-track, then the engine will be screaming at the finish line. Using higher gears helps the nitrous work better under a load and also helps keep the engine in its powerband all the way down the track.

Here's a reason low gears can be beneficial to dragstrip performance that you've probably never considered. Assume that a car is in a 1:1 trans ratio, has 26-inch-tall tires and 3.08:1 axle gears. When accelerating from 50 to 70 mph, engine speed increases by about 800 rpm. Put 4.10:1 gears in the same car, and engine speed increases by 1060 rpm—the difference is 40 rpm per mph with the 3.08s versus 53 rpm per mph for the 4.10s. The greater rate of rpm increase versus road speed provides greater acceleration. Since horsepower increases as engine rpm increases (up to the point when the torque curve drops off at a greater rate than engine rpm increase), the engine is able to overcome loads more easily with lower gears than with higher gears. This helps not only in acceleration but in maintaining road speed under a load such as when climbing a steep grade.

If you prefer top speed to dragstrip performance, then higher axle gears, like 2.76:1 or 3.08:1, may be called for. The higher gears will reduce engine rpm versus road speed. Another way to look at it is that the car will go faster at the engine's rpm limit than it would with lower gears. Low-speed acceleration will suffer, but that can be cured with one of today's manual five- or six-speed trannies that provide lower First and Second gears for acceleration and also feature overdrive for even more top speed—or engine-rpm reduction, depending on how you look at it.

Why Does Tire Height Affect Cruise RPM?

Sometimes you'll hear people talk about "effective gear ratio" to explain the drop in cruise rpm after installing taller tires or the increase in rpm with shorter tires. Here's their theory: If a car starts with 3.50:1 gears and 26-inch-tall tires, but the tires are then swapped to 30-inchers, then the effective gear ratio is 3.08:1. In other words, the cruise rpm with 3.50:1 gears and 30-inch tires is the same as it would be if the 26-inch tires were retained and 3.08:1 gears were installed.

We don't like this concept because it's complicated and irrelevant. You can't walk up to a car at cruise night and calculate it's "effective gear ratio" unless you know its original tire size. Many people will say, "It's got 3.73:1 gears, but they act like 3.50s because the tires are taller." Taller than what? There is no standard from which to compare. Besides, "effective gear ratio" implies that a ratio has been changed, but tire size has no effect on axle ratio at all. Here's proof: If you have 4.10:1 gears, then the driveshaft will turn 4.1 times for each revolution of the tires regardless of their size.

However, changes in tire diameter do affect the car's cruise rpm, and perhaps its acceleration, because you've altered the number of tire revolutions per mile. For example, a tire with a true diameter of 26 inches has a circumference of 81.68 inches; a tire 30 inches tall has a circumference of 94.25 inches. That means each time the 30-inch tire completes one revolution it will move the car about 12-1/2 inches farther than the one revolution of the 26-inch tires. Therefore, the taller tire requires less input rpm (engine speed) to travel the same distance. Conversely, shorter tires require more engine speed per mph. That's why shorter tires seem to act like lower axle gears, and taller tires seem to act like higher gears.

There are two other reasons taller tires can tend to reduce acceleration. First, taller usually means bigger, which means heavier. Secondly, taller tires have a greater static loaded radius, or the distance from the center of the axleshaft to the ground when the tire is installed at operating pressure and loaded with the weight of the vehicle. The greater the static loaded radius, the greater the length of the lever between the axle and the ground, the greater the tire's ability to resist the acceleration of the car. However, taller tires also have a larger contact patch than shorter tires, so the dragstrip tractive advantages usually outweigh any disadvantages of taller tires, especially when the proper axle gears are chosen to compensate for the tire size.